At present, image forming apparatuses are demanded of higher printing speeds and higher image qualities of printed images. For this purpose, a semiconductor laser driving device in a laser beam printer or the like must increase the driving frequency of the semiconductor laser element and increase the number of semiconductor laser elements.
In general, the laser driving circuit is made up of a semiconductor laser element which forms a light-emitting circuit, a laser driving semiconductor device, a wiring which connects the semiconductor laser element and laser driving semiconductor device, a main power transmission wiring, a main GND (ground) wiring, a wiring which connects the main power transmission wiring and light-emitting circuit, and a wiring which connects the main GND wiring and light-emitting circuit. The light-emitting circuit supplies a current of about several ten mA at a frequency of about 10 MHz to the semiconductor laser element, flickers the semiconductor laser, and thereby converts a received electrical data signal into an optical data signal.
However, if a large high-frequency current flows through the semiconductor laser driving circuit, a noise component is generated in a current flowing through the light-emitting circuit. That is, a noise component such as ringing is generated in the light-emitting circuit due to the impedances of the semiconductor laser element, laser driving semiconductor device, main power transmission wiring, main GND wiring, and remaining wirings. The generated noise component propagates as a noise current to the main power transmission wiring and main GND wiring. The noise current degrades the quality of the emission current of the semiconductor laser element, and decreases the precisions of the emission timing and emission amount, obstructing the demand for higher image qualities. When the power cable shares GND (ground) with another signal cable or the like, the noise current may also flow into the signal cable.
When the laser driving semiconductor device supplies/stops a high-frequency current, the amount of current flowing through the main power transmission wiring varies. In response to this, the power supply voltage varies, generating radiation noise. In addition, the voltage level of the main power supply instantaneously varies due to the above-mentioned noise component such as ringing, generating larger radiation noise. These problems of radiation noise become more serious when the main wiring is connected to a power cable. Large radiation noise is radiated from the power cable functioning as an antenna source.
As a method of solving these problems, Japanese Patent Laid-Open No. 63-044782 discloses a method of arranging a filter on a wiring which connects a semiconductor laser element and laser driving semiconductor device. However, as the driving frequency increases, degradation of the emission current waveform by the filter itself stands out. It is very difficult to simultaneously satisfy increases in speed and image quality and reduction of radiation noise.
Recently, a compensation circuit is generally added as a method of suppressing a noise component. FIG. 12 shows an example of a laser driving circuit to which the compensation circuit is added.
A compensation circuit 10 made up of a compensation element 11, wiring 12, and compensation semiconductor device 13 is parallel from a main power transmission wiring 2 with a light-emitting circuit 6 made up of a semiconductor laser element 7, wiring 8, and laser driving semiconductor device 9. The compensation circuit 10 is driven complementarily with the light-emitting circuit 6, and implements a compensation function using a current from a feed capacitor 1 as a constant current. Since a current flowing through the main power transmission wiring 2 is a constant current, variations in power on the main wiring can be suppressed.
Ringing generated in the compensation circuit 10 is opposite in phase to ringing generated in the light-emitting circuit 6. At a branch point 3 between a first wiring 4 and a second wiring 5, ringing of a current flowing through the light-emitting circuit 6 is canceled by ringing of a current flowing through the compensation circuit 10. As a result, generation of ringing can be suppressed, and the quality of the emission current of the semiconductor laser element in the light-emitting circuit 6 can be increased to implement higher-precision emission.
However, in the laser driving circuit shown in FIG. 12, the light-emitting circuit 6 and compensation circuit 10 must exhibit the same electrical characteristic. In other words, if the electrical characteristics of the light-emitting circuit 6 and compensation circuit 10 even slightly deviate from each other, noise components (e.g., ringing) which should be canceled are adversely added to generate a larger noise component. To prevent this, an impedance Z4 of the wiring 4 and an impedance Z5 of the wiring 5 must be designed equally. Alternatively, the wirings 4 and 5 are designed as short as possible to decrease the values of the wiring impedances Z4 and Z5 to 0 as much as possible.
However, the position of the semiconductor laser element is determined preferentially to the position of an optical system for processing a laser beam output from the semiconductor laser element. Since the degree of freedom of the wiring is greatly limited due to the demand for higher image qualities, it is very difficult to arbitrarily design the lengths of the wirings 4 and 5 and the like.
These days, the number of semiconductor laser elements, which is conventionally one or two for each color, is increasing to four owing to the demand for higher image qualities. The numbers of components and wirings arranged on the laser circuit board greatly increase. On the other hand, the optical axis of the laser beam is stabilized by fixing the laser circuit board to a surrounding metal housing. If the board size is increased for a higher degree of freedom of the wiring, the vibration resistance decreases, and the laser circuit board tends to shake along the optical axis, failing to meet the demand for higher image qualities.
Since the semiconductor laser element 7 and compensation element 11 are not formed from completely identical components, an impedance Z6 of the light-emitting circuit 6 and an impedance Z10 of the compensation circuit 10 have slightly different values. The impedance difference is not problematic at a driving frequency of about 10 MHz, but poses a serious problem at present because higher speeds up to about 60 MHz are demanded. For this reason, the laser driving circuit shown in FIG. 12 cannot satisfactorily meet the demand for higher driving frequencies.